Direct Observation of Polyethylene Shish-Kebab Crystallization Using in-Situ Atomic Force Microscopy J. K. Hobbs* and M. J. Miles H. H. Wills Physics Laboratory, Tyndall Avenue, Bristol BS8 1TL, UK Received September 29, 2000 Revised Manuscript Received November 25, 2000 Introduction. The crystallization of polymers into oriented structures, caused by extensional flow fields in the melt, has been the subject of extensive study for many years. 1-4 Recent advances in atomic force micros- copy (AFM) have allowed samples to be heated and imaged in situ, in real time, with nanometer resolution. This has already been applied to polymer crystalliza- tion, 5-8 following earlier work in our laboratory on the room temperature crystallization of poly(hydroxybu- tyrate-co-valerate) (PHB/V) 9 using AFM. To date, all the AFM studies have dealt with quiescent melts, in which spherulitic growth dominates. However, it is well-known that under normal polymer processing conditions the melt is submitted to severe shear and extensional flow fields, frequently resulting in oriented microstructures. In this communication we present our preliminary AFM results imaging the crystallization of preprocessed melts, revealing directly, for the first time, the lamellar scale growth as it occurs from the oriented molecular backbone. From observations such as these, significant new insights into the mechanisms underlying the formation of morphologies that occur during industrial polymer processing can be gained. Here our intention is to place this new observation on the map, leaving a fuller presentation and discussion of the work to a future publication. Experimental Details. A sharp polyethylene frac- tion was used, supplied by the National Bureau of Standards, M w 119 600, M w /M n 1.19. A 1% suspension of the polymer in p-xylene was prepared by dissolving for 20 min at 120 °C and quenching to room tempera- ture. A drop of the suspension was then placed on a glass coverslip on a hot bench at 150 °C and held in the melt for 2 min. The resulting thin film was then quenched to room temperature. This film was remelted on a Linkam hot stage at 160 °C for 2 min and cooled to 145 °C. A razor blade was dragged across the glass coverslip in order to cause oriented crystallization in the melt. When this process was observed using an optical microscope, it was clear that the melt started to crystal- lize as soon as the blade was brought into contact with the melt, due to the drop in temperature that this caused. A highly oriented birefringent area was formed where the razor blade had sheared and extended the melt. As soon as the razor blade was removed, the unoriented regions remelted, but the high birefringence remained in the oriented part of the film. The Linkam was cooled to 135 °C and then moved into position under the scan tube of a Digital Instru- ments D3100 AFM. To protect the piezoelectric crystal from the heat, a sheet of aluminum foil and a sheet of Kapton were placed between the scan tube and the heater, the cantilever projecting through a small hole in these protective sheets. The AFM was operated in Tapping mode, and phase, height, and amplitude images were collected simultaneously. Imaging conditions were maintained so as to just allow the surface to be tracked while maintaining the fast scan rates necessary to follow the process. All images were taken at 256 × 256 pixels. Owing to the proximity of the (unheated) cantilever and the heat shield to the sample surface, a temperature difference of ∼7 °C was estimated between the nominal value given by the Linkam and that of the sample surface. However, the sample was being cooled, and we have not been able to accurately calibrate the Linkam- AFM system on cooling due to slight differences between each experimental setup. The temperatures quoted in the rest of this paper are those given on the Linkam * Corresponding author. Tel 44 (0)117 9288747; Fax 44 (0)117 9255624; e-mail jamie.hobbs@bristol.ac.uk. Figure 1. Two AFM phase images showing dormant shish- kebab crystals at a nominal temperature of 135 °C. Black to white represents a change in phase angle of 30°. (b) is a ×1.27 software zoom. Scale bars represent 300 nm. 353 Macromolecules 2001, 34, 353-355 10.1021/ma001697b CCC: $20.00 © 2001 American Chemical Society Published on Web 12/19/2000